It doesn't. That's an input field, you can edit it and go back to -4000.
You are correct, The UI doesn't show it very clearly tho.
the ui shows false information when u pick a modern date
yeah I noticed it in the URI before I found it in the UI.
Well, currently the data really starts at 1800 though. For earlier dates data is filled very sparingly, that’s why, I suppose, it defaults to 1800.
Also 1800s has good coverage of sources. Like this one in Europe: https://maps.arcanum.com/en/ (actually this one is probably licensed - but some of those maps have public domain version).